Coal liquefaction at staged temperatures

ABSTRACT

IN THE SOLVENT LIQUEFACTION OF SLURRIED CAKING-TYPE COAL SOLIDS AT ELEVATED DEPOLYMERIZING TEMPERATURES ABOVE 700* F., THE FORMATION OF UNCOVERTIBLE COAL RESIDUES IS REDUCED BY FIRST FORMING A SUBSTANTIALLY COMPLETE DISPERSION OF THE COAL IN THE SOLVENT BEFORE EXPOSING THE COAL TO DEPOLYMERIZING TEMPERATURES. FOR EXAMPLE, THE SLURRY IS FIRST MAINTAINED AT TEMPERATURES WITHIN THE RANGE FROM ABOUT 500* F. TO ABOUT 700*F. WHILE BEING AGITATED UNTIL THE VISCOSITY OF THE SLURRY, AFTER HAVING INCREASED TO A MAXIMUM, FALLS FROM THE MAXIMUM TO A VISCOSITY WITHIN A PREDETERMINED RANGE LOWER THAN THE MAXIMUM WHICH INDICATES THE FORMATION OF A SUBSTANTIALLY COMPLETE DISPERSION. THEREAFTER, THE DISPERSED SLURRY IS HEATED AT DEPOLYMERIZING TEMPERATURES ABOVE 700*F. IN INTIMATE CONTACT WITH FREE RADICAL CHAIN TERMINATORS SUCH AS PROVIDED IN A HYDROGEN-DONOR SOLVENT, RESULTING IN THE MARKEDLY REDUCED FORMATION OF INTRACTABLE POLYEMRS.

Sept. 19, 1972 E. L WILSON ETAL 3,692,662

COAL LIQUEFACTION AT STAGED TEMPERATURES Filed Oct. 9, 1970 v s Sheets-Sheet 1 INFLUENCE OF TEMPERATURE'ON VISCOSITY 33 WT./ COAL/HYDROGENATED CREOSOTE OIL SLURRY o l l l I I I 1 Non- Newtonian O 'E Newtonian 2 l0.0- i- 3 Q) IOO Min'-l I08 Min. 2 Heating p a. I o o o 0 \U\ \I W 0 8 30- I 0 30 Q Min. i k

0 an: 2.0 I Coal Type Illinois No.6 ggug Coal Size IOO Mesh I Moisture Content- Dry Shear Rate 868 Sec.

05 l l I v l l TEMPERATURE, F.

INVENTORS.

EDWARD L. WILSON, ROBERT E. PENNINGTON, BY

Sept. 19, 1972 \MLSON ET AL COAL LIQUEFACTION AT STAGED TEMPERATURES 5 Sheets-Sheet 3 Filed Oct. 9, 1970 2.5125 mm h. 009 v I. @N PM mm mm ll mm 5000702 20 6 m d 002-09 mm 262cm mwooxw INVENTORS. WILSON,

EDWARD L. ROBERT E PENNINGVTON, fwo' z' 'yw AT ORNEY.

United States Patent Office 3,692,662 Patented Sept. 1.9, 1972 3,692,662 COAL LIQUEFACTION AT STAGED TEMPERATURES Edward L. Wilson and Robert E. Pennington, Baytown,

Tex., assignors to Esso Research and Engineering Com- Filed Oct. 9, 1970, Ser. No. 79,609 Int. Cl. Cg 1/04 U.s. Cl. 208-8 9 Claims ABSTRACT OF THE DISCLOSURE In the solvent liquefaction of slurried caking-type coal solids at elevated depolymerizing temperatures above 700 F., the formation of unconvertible coal residues is reduced by first forming a substantially complete dispersion of the coal in the solvent before exposing the coal to depolymerizing temperatures. For example, the slurry is first maintained at temperatures within the range from about 500 F. to about 700 F. while being agitated until the viscosity of the slurry, after having increased to a maximum, falls from the maximum to a viscosity within a predetermined range lower than the maximum which indicates the formation of a substantially complete dispersion. Thereafter, the dispersed slurry is heated at depolymerizing temperatures above 700 F. in intimate contact with free radical chain terminators such as provided in a hydrogen-donor solvent, resulting in the markedly reduced formation of intractable polymers.

BACKGROUND OF THE INVENTION This invention relates to the conversion of coal solids to liquids, and more particularly, to the liquefaction and hydrogenation at elevated temperatures of coal slurried in liquid vehicles. More specifically, the invention involves a method of reducing the formation of intractable coal residues formed in the solvent liquefaction of caking-type coals dispersed in a hydrogen-donor solvent at coal depolymerizing temperatures above about 700 F.

In processes designed to produce liquid hydrocarbon products from coal, oils derived from coal are typically used to slurry comminuted coal to carry the coal into high temperature, high pressure reactors where liquefaction of convertible coal occurs. The oils that generally are selected for this purpose contain partially hydrogenated polynuclear aromatics such as tetrahydronaphthalene and its homologs, for these molecules have the ability to donate hydrogen to the liquids being made from the coal at the elevated temperatures used in liquefaction reactors. Liquids are made from the coal when weaker chemical bonds in the very large coal molecules are thermally cracked. (Oxygen-to-carbon bonds found in either linkages are an example of weaker bonds that are broken.) Free radicals result when the bonds of the coal molecules are broken. If a free radical chain terminator such as hydrogen from a hydrogen-donor solvent is available to stabilize the free radicals, desirable lower molecular weight hydrocarbons soluble in benzene, are produced. On the other hand, if hydrogen is not available to stabilize the free radicals, free radicals combine with one another through carbon-to-carbon bonds, producing high molecular weight molecules that are more stable and intractable than the parent coal molecules. These high molecular weight molecules, many of which are benzene insoluble, are essentially unconvertible in the liquefaction reactor and require a considerable expenditure of energy in further processing in order to reduce them to a suitable molecular weight range.

In prior art solvent liquefaction processes, the slurried coal is introduced into the liquefaction reactor in a substantially indispersed condition. As a result, when the coal depolymerizes and forms free radicals, hydrogen availability is sufficiently poor that a substantial amount of the free radicals recombine to form significant quantities of the unwanted high molecular weight molecules.

By the process of this invention, the coal is dispersed and in intimate contact with free radical chain terminators when coal molecules are depolymerized so the unwanted recombination happening in prior processes is avoided, making it possible to maximize the production of hydrogen stabilized molecules.

SUMMARY OF THE INVENTION We have discovered that more benzene soluble product and less intractable and essentially unconvertible coal residues are formed in the solvent liquefaction of slurried caking-type coal particles when the coal is treated to substantially completely disperse the coal in the solvent before subjecting the coal to temperatures at which free radicals are formed by depolymerization. This treatment involves maintaining the coal slurry in a temperature range below 700 F. under agitation for a time period long enough to allow such a dispersion to occur. Dispersion is accompanied by a distinctive phenomenon: the viscosity of the slurry first increases to a maximum and then falls from that maximum to viscosities which are even lower than attained before the viscosity increase began. Substantially complete dispersion is evidenced when the viscosity of the slurry has fallen from the maximum into a predetermined range lower than the maximum, preferably a range extending from an upper limit equal to the viscosity of the slurry at room temperature, before the slurry is heated, to a lower limit equal to the viscosity of the slurry at 700 F., after heatup of the slurry through the viscosity maximum. When substantially complete dispersion has occurred, the dispersed coal slurry is then heated at depolymerizing temperatures in a range above 700 F. in intimate contact with free radical chain terminators such as provided in a hydrogen-donor solvent. The dis persed slurry is held at the depolymerizing temperatures until a predetermined conversion of the coal to benzene solubles occurs.

The invention is preferably carried out using a hydrogen-donor solvent as a source of hydrogen for terminating free radical chains. In this form, a slurry of coking coal in a hydrogen-donor solvent is maintained at temperatures preferably within the range from about 500 F. to about 700 F. under agitation. Holding the coal at these temperatures causes the coal to disintegrate and dissolve without a significant number of the coal molecule bonds being broken to form free radicals. The slurry is held at these temperatures under the agitation supplied until the convertible portions of the coal are substantially uniformly dispersed in the hydrogen-donor solvent. When suitable dispersion is indicated, for example, by viscosity measurements conducted on the slurry as more fully described below, the temperature of the slurry is increased to bond-breaking or depolymerizing temperatures above about 700 F., suitably within the range from about 700 F. to about 950 F. under a pressure elfective to maintain the dispersed slurry substantially in liquid phase, suitably a pressure within the range from about 350 p.s.i.g. to about 3500 p.s.i.g. In this second temperature stage, the dissolved coal particles are well dispersed in the hydrogen-donor solvent and the chance of a hydrogendonor stabilization of free radicals generated by bondbreaking is maximized. At the same time, the chance for free radicals to combine with one another to produce undesirable molecules is minimized. The dispersed slurry is maintained at the elevated temperatures above 700 F. until a predetermined conversion of the coal to benzene solubles is obtained. For example, where the solvent/coal ratio is within the range from about 0.1:1 to about 2:1, and where the hydrogen-donor solvent boils within the range from about 300 F. to about 900 F. and contains at least about 30 weight percent of hydrogen donors, preferably at least 50 weight percent, the dispersed slurry is maintained at the elevated temperature range until at least 80 percent conversion of the coal to benzene solubles is obtained. Suitable conversion levels are obtained, however, by maintaining the dispersed slurry at the elevated temperatures above 700 F. for from about 5 to about 60 minutes.

As indicated, dispersion is distinctly evidenced by viscosity changes which the slurry undergoes during heating at temperatures below 700 F. These changes are illustrated by the following examples.

EXAMPLE 1 One part of Illinois #6 coal was slurried in two parts of hydrogenated creosote oil and the viscosity of the slurry was measured at various temperatures. The results of this test are shown in FIG. 1, which is a plot of a slurry viscosity in centipoise, against temperature. Heatup times are inscribed upon the face of the drawing. Actual data collection was started at about 248 F. The viscosity at 70 F. was previously determined and is given as a reference value.

As shown in FIG. 1, the viscosity of the slurry decreased continuously as the slurry was heated from 78 F. to 493 F. and then remained relatively constant up to about 600 F. As indicated in FIG. 1, the slurry behaved as a Newtonian fluid up to about 600 F. Above 600 F. and up to about 660 F., the viscosity of the slurry increased sharply after an initial hesitation, and passed through a maximum at about 660 F. Then the viscosity decreased precipitously with increasing tempera-.

ture up to 695 R, where the run was terminated. Thereafter, the slurry was cooled down. As indicated in FIG. 1, the rheological behavior of the slurry changed from New tonian to non-Newtonian at a temperature of about 635 F. when the viscosity of the slurry began to sharply increase. However, after passing through the viscosity maximum, the behavior of the slurry, beginning at about 680 F. was that of a non-Newtonian fluid. As illustrated by FIG. 1, the viscosity of the cooling slurry was lower at 565 F. than the viscosity of the slurry at that temperature when it was being heated up. There was no solvent loss by evaporation during the run. This illustrates a phenomenon generally observed in these experiments, that dispersion of the coal is evidenced by the fact that the viscosity of a suitably dissolved and mixed slurry, which has passed through its maximum viscosity and fallen below the low viscosity value it had before beginning its viscosity climb, will be lower, on cooling, at any temperature, than the viscosity of the slurry before suitable dispersion has occurred.

EXAMPLE 2 A determination was made of the influence of temperature upon the viscosity measured at 868 secsof a 4 slurry of one part Illinois #6 coal in 1.2 parts hydrogenated creosote oil. The results are illustrated in FIG. 2. As depicted on the face of FIG. 2, the slurry was maintained at 400 F. for one hour and then heated to 675 F. in about 50 minutes. The viscosity of the slurry decreased with increasing temperature up to about 560 F., but then increased sharply as the temperature rose to 675 F., going off scale for 5 minutes even at the high shear rate of 868 secs- Dropping back on scale, the viscosity of the slurry at 675 F. temperature fell sharply. In two minutes it was close to the viscosity it had when it began its climb. After 8 minutes, the viscosity was well below the value it had earlier at 560 F., when the viscosity of the slurry began its meteoric climb. The viscosities of the cooling slurry were lower, at the same temperatures, than the viscosities of the slurry during the heatup period.

In this study, in order to correlate the changes in viscosity during heating through the 500--700 F. range with physical changes undergone by the slurried particles, samples of coal particles were collected at the beginning of the high viscosity range (Sample A), at the maximum viscosity (Sample B), and at a point where the viscosity was lower than immediately before the climb to the maxi mum (Sample C).

Microscopic examinations of Sample A showed no evidence of coal dissolution. Small particles were in abundance, and the edges of larger particles were intact. The appearance of small cavities in the coal particles indicated gas generation and subsequent coal swelling was just beginning to occur. Ash analysis of the solids indicated that no coal conversion had occurred.

Microscopic examination of Sample -B showed that some large particles had developed gas vacuoles or cavities indicating softening and gas bloating. Edge destruction was evident and a smaller quantity of fine particles than occurred in Sample A were observed, indicating that coal dissolution had begun. Ash analysis of the solids indicated that no coal conversion or upgrading had occurred.

Microscopic examinations of Sample C showed that the coal had been completely dispersed: no identifiable coal particles existed. The sample contained reagglomerated material. It appeared that the coal dispersed as fragments of collodial material which subsequently reagglomerated on cooling for microscopic examination. Ash analysis of the solids indicated that less than 7 percent of the coal was converted to benzene soluble material. These data show that dissolution or dispersion of the coal occurred with only minor or insignificant amounts of chemical bond-breaking.

Taken together, the physical state and analyses of these three samples show that the sharp increase in viscosity of the slurry as the temperature is raised is due to a swelling of the coal particles. This swelling is apparently the initial stage of the coal disintegration/dissolution process. As the coal disintegration and dissolution proceeds, the viscosity goes through a maximum and decrease again.

The lower the viscosity after decreasing from the maximum, the more complete the dispersion. By correlating viscosities on the downhill side from the maximum with the particular heating conditions employed on a slurry of a particular caking-type coal in a selected solvent at whatever coal/solvent ratios are used, one can determine the residence time for those conditions necessary to provide a substantially completely dispersion of the coal particles. In some instances, a suitable such dispersion may occur at a downhill viscosity that is higher than the viscosity of the untreated slurry at room temperature, i.e., the viscosity the slurry had before heatup to eifect the dispersion. In most instances, however, a preferable viscosity range within which an adequate dispersion is evidenced extends from an upper viscosity about equal to the viscosity of the slurry at room tem perature, before heatup through the maximum, to the viscosity of the slurry at 700 F., after heatup through said maximum. In any event, attainment of heating conditions below 700 F. necessary to get an adequate dispersion has occurred if, after heating, the viscosity of the slurry, measured at any temperature below the temperature Where the upswing in slurry viscosity began, is less than the viscosity of the unheated slurry at that temperature, as illustrated in both FIGS. 1 and 2.

A more complete understanding of the invention is provided by a detailed description of a preferred mode of carrying it out, as illustrated in the schematic flow diagram of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 3, a caking coal in particulate form is fed by way of line into a mixing zone 11, where it is mixed with a hot hydrogenated recycle oil boiling from about 400 F. to about 700 F. and introduced by way of line 12. The resultant slurry formed in the mixing zone under the influence of a stirrer 13 has a temperature of about 350 F. The rates of feed through lines 10 and 12 are adjusted so that the solvent/coal ratio in the mxing zone is about 1.2:1.

The slurry is conducted from the mixing zone by way of line 14, and is mixed with about 1-3 weight percent of hydrogen introduced into line 14 from line 15, being then conducted through a first preheater 16, which heats the slurry up to about 700 From first preheater 16, the slurry is discharged into a first-stage liquefaction reactor 17. While being agitated by stirrer 18, it is maintained at a temperature within the range from about 500 F. to about 700 until the viscosity of the slurry passes through a maximum value to a predetermined viscosity lower than the viscosity of the slurry at room temperature, before heatup in preheater 16, preferably at least until the viscosity falls to the viscosity value it had just before it began its climb to its maximum viscosity. Thereafter, the slurry is withdrawn from the first liquefaction reactor 17 and conducted by way of line 19 through a second preheater 20, which heats the slurry above 700 F. to a temperature range effecting coal depolymerization, suitably up to about 950 F. The dispersed, heated slurry is then discharged iato a second liquefaction reactor 21. In reactor 21, the well dispersed coal molecules are maintained at the depolymerizing temperatures under a pressure within the range from about 350 p.s.i.g. to about 3500 p.s.i.g. suitable to maintain at least 80 percent of the solvent in the liquid phase. At these elevated temperatures, bond-breaking occurs in the coal molecules, forming free radicals. The close proximity of the free radicals to hydrogen permits their stabilization by hydrogen, with consequent formation of the desirable lighter coal molecules. After a residence time sufficient to permit conversion of coal molecules to lighter molecules, expressed as the formation of benzene solubles, preferably after a residence time suflicient to permit the formation of at least 80 percent benzene solubles, based on the DAP feed coal, the raw liquefaction product from liquefaction reactor 21 is withdrawn by line 22 and charged to a fractionating tower 23, from which a naphtha stream is removed by way of line 24, a 400/700 F. fraction removed by way of line 25, and a 700 F.+ fraction removed by way of line 26. The 700 F.+ fraction is then distilled in a tower 27 to recover overhead a gas oil fraction boiling from 700 F. to 1000" F. by way of line 28. A heavier bottoms fraction is withdrawn from unit 27 by way of line 29.

A portion of the 400/700 F. fraction withdrawn from fractionating tower 23 line 25 is suitably diverted by line 31 and upgraded in a solvent hydrogenation zone 27 for ultimate recycle to mixing zone 30, by line 12. The solvent hydrogenation in zone 27 is suitably carried out at a temperature within the range from 650 F. to 850 F., preferably 7 00 F, a pressure within the range from 650 p.s.i.g. to 2000 p.s.i.g., preferably 1350 p.s.i.g., a space velocity within the range from 1 to 6 w./hr./w., preferably about 2 w./hr./w., and a hydrogen-treat rate of 1,000 to 10,000, preferably 5,000 s.c.f. of hydrogen per barrel of oil feed from line 26. Suitably the catalyst is a hydrogenation catalyst such as cobalt molybdate.

The basic feedstock to the process just described is a solid, particulate coal such as a bituminous coal, subbituminous coal, lignite, brown coal, or a mixture thereof, which has caking properties. A typical proximate and ultimate analysis of an Illinois #6 Coal, which is a caking coal, is set out in Table I, which follows:

TABLE I Chemical analysis of coal weight percent dry analysis Illinois #6 Coal (with confidence limit) Carbon 68.77i0.22 Hydrogen 5.01:0.03 Organic oxygen 9.80: Nitrogen 1.13:0.03 Organic sulfur 3.13:0.78 Total sulfur 4.51:0.10 Ash 10.23i0.l3 Mineral matter ll.7i0.l Volatile matter (DMF basis) 426310.47

It is desirable to grind the coal to a particle size distribution of from about 8 mesh and finer. However, it has been found that the liquefaction process of this invention can be adequately conducted even if particles as large as one-quarter inch on the major dimension are employed.

The coal preferably is dried to remove excess water, although it is feasible to utilize coal which is not moisture free if the liquefaction facilities are sized to allow the withdrawal of evolved steam. The coal can be dried by conventional techniques prior to mixing it with the hydrogendonor solvent to form the slurry feed for liquefaction. It is preferred, however, to mix the wet coal with a hot hydrogen-donor oil in a mixing zone to volatilize water in the mixing zone, thereby reducing the feed slurry moisture content to less than about 2 weight percent.

The hydrogen-donor solvent which is mixed with the coal feedstock in the mixing zone, suitably at a solvent/ coal weight ratio of from about 0.8:1 to about 2:1, suitably boils within the range from about 300 F. to about 900 F., and more preferably within the range from 400 F. to 800 P. so that it remains substantially in the liquid phase at the elevated temperatures employed in the liquefaction zone. Desirably, the solvent contains at least 30 weight percent, preferably at least 50 Weight percent, of compounds which are known to be hydrogen donors under the conditions used in the liquefaction zone. The hydrogen-donor solvent stream suitably contains one or more hydrogen-donor compounds in admixture with nondonor compounds or with one another. Preferred hydrogen-donor compounds include such compounds as indane, C -C tetrahydronaphthalenes, C and C acenaphthenes, di-, tetra-, and octahydroanthracene, and tetrahydroacenaphthene, and other derivatives of partially saturated Polynuclear aromatic compounds.

Preferably, the solvent stream is a hydrogenated recycle solvent fraction as in the preferred embodiment. The composition of such a fraction varies somewhat, depending upon the source of the coal used as the feedstock to the system, the operating conditions and the overall process, and the conditions used in hydrogenating the solid fraction for recycle after liquefaction. However, a typical description of a hydrogenated recycled solvent fraction will be similar to that shown in Table II.

TABLE IL-TYPIOAL RECYCLE SOLVENT Distillation-glass spiral Cum. wt., Sp. gr., 760 mm. percent over 60 F./60 F.

Mass spec. analysis Weight No. rings Typical compound percent 011112114 5. 4: C Hqn-(i Alkylbenzene 2. 5 2 CuH2n8 Tetra 30. 5 2.- Cullen- Indene 8. 2 2-. CnHinlZ Naphthalene.-." 15. 4 3 CnHzn-M Acenaphthene 17. 1 3 CnH2n16 Aeenaphthylene 8. 4 3---- CnH -18 Phananthrene 7. 7 4. (Julian-20 Naphthenoanthracene 2. 2 4.- C EM-22 Pyrene 2. 6 4. CHHWZ I Chrysene 0 5-.. CnH2n'2fi Cholanthrenes- O 5 C ms-28 Benzopyrenes 0 Total 100. 0

% Conversion=100 m wherein a=percent ash in the moisture-free coal feedstock, S =percent solids in the feed slurry, S =percent solids in the product slurry.

Before the percent of solids in the product slurry is measured, benzene is added to the product slurry as a solvent, in a 1:1 volume ratio.

Having now fully and particularly described our invention, including a preferred mode of carrying it out, what is desired by Letters Patent is defined by the ap-. pended claims.

We claim:

1. In the solvent liquefaction of a caking coal wherein the coal is treated in a slurry with said solvent, the improvement which comprises:

maintaining said slurry in a holding zone under agitation in a temperature range below 700 F. effective to cause said coal to substantially completely disperse in said solvent, until the coal is substantially completely dispersed in said solvent, and

thereafter heating said dispersed coal slurry, in intimate contact with a free radical chain terminator, to a temperature above 700 F. at which substantial depolymerization of coal is accomplished. 2. In the solvent liquefaction of a caking coal wherein the coal is treated in a slurry with said solvent, the improvement which comprises:

maintaining said slurry in a holding zone under agitation in a temperature range within the range from about 500 F. to about 700 F. effective to cause the viscosity of the slurry to pass through a maximum for such temperature range, until after said slurry viscosity passes through said maximum, and

thereafter heating said coal slurry, in intimate contact with a free radical chain terminator, in a temperature range above 700 F. at which depolymerization of the coal is accomplished.

3. In the solvent liquefaction of a caking coal wherein the coal is treated in a slurry with a hydrogen-donor solvent containing at least 30 Weight percent of hydrogendonor compounds, the improvement which comprises:

maintaining said slurry in a holding zone under agitation for sufficient time in a temperature range within the range from about 500 F. to about 700 F. effective to cause the viscosity of the slurry to pass through a maximum for such temperature range to attain a viscosity lower than said maximum indicative of a substantially complete dispersion of said coal in said solvent, and

thereafter heating the dispersed coal slurry in a temperature range above 700 F. at which depolymerization of the coal is accomplished.

4. In the solvent liquefaction of a caking coal wherein the coal is treated in a slurry with a hydrogen-donor solvent containing at least 30 weight percent of hydrogen donor compounds, the improvement which comprises:

maintaining said slurry in a holding zone under agitation for sufiicient time at temperatures Within the range from about 500 F. to about 700 F. effective to cause the viscosity of the slurry, after having increased to a maximum, to fall from said maximum to a lower viscosity which is indicative of a substantially complete dispersion of said coal in said solvent, and

thereafter heating said slurry to temperatures within the range from about 700 F. to about 950 F. under a pressure within the range from about 350 p.s.i.g. to about 3500 p.s.i.g. effective to maintain the solvent substantially in liquid phase until the coal is converted to a predetermined percent benzene solubles, based on DAF feed coal, whereby the formation of unconvertible coal residues is reduced.

5. The method of claim 4 in which said viscosity of said dispersed coal slurry, after passage through said maximum, is lower on a cooling of such dispersed coal slurry to a lower temperature in said temperature range than before passing through said maximum.

6. The method of claim 4 in which said predetermined range extends from an upper viscosity value equal to the slurry viscosity at room temperature after heatup, to the viscosity of the slurry at 700 F., after heatup and descent from said maximum.

7. The method of claim 4 in which said slurry is maintained at said temperatures within the range from about 500 F. to about 700 F. for from 25 to about 60 minutes.

8. The method of claim 3 in which said lower viscosity is not greater than the minimum viscosity of said slurry in said temperature range prior to passage of the viscosity of the slurry through said maximum.

9. In the solvent liquefaction of a caking coal, wherein the coal is treated in a slurry with a hydrogen-donor solvent containing at least 30 weight percent of hydrogen- 10 donor compounds, the improvement which comprises: References Cited maintaining said slurry in a holding zone in a temperature range from about 500 F. to about 700 F. UNITED STATES PATENTS under agitation until said slurry acts as a Newtonian 522 2 1 fluid, indicative of a substantially complete dispersion 5 of said coal in said solvent, and thereafter heating the dispersed coal slurry in a tem- DELBERT GANTZ Pnmary Exammer perature range above 700 F. at which depolymeriza- V. OKEEFE, Assistant Examiner tion of the coal is accomplished. 

